async vs co
Which should you use? A side-by-side look at what each one does, who it suits, what it costs, how hard it is to set up, and where each falls short.
Open source · MIT
A JavaScript utility library of helper functions for running asynchronous code in Node.js or the browser.
Open source · MIT
A small JavaScript helper for running generator functions as async control flow.
The short answer
Choose async if you need…
- JavaScript developers who need to run many asynchronous operations over a collection of items
- Node.js projects that use callback-style code and want helper functions for it
- Developers who want to limit how many asynchronous tasks run at the same time
- Projects bundling with Webpack or Rollup that can use the included ESM build
Choose co if you need…
- Node.js developers maintaining code that already uses generator functions
- Developers reading or migrating older async code written around co-style generators
- Developers who want a minimal helper rather than a full framework
Side-by-side
| async | co | |
|---|---|---|
| Type | Library | Library |
| Price | Free and open source | Free and open source |
| Licence | MIT | MIT |
| GitHub stars | 28.1k | 11.8k |
| Setup difficulty | Intermediate | Intermediate |
| How you get it | Install with a package manager | Install with a package manager |
| Main language | JavaScript | JavaScript |
| Last code update | 5 days ago | 6 years ago |
What each one is
async
Async is a JavaScript utility module that provides functions for working with asynchronous code, such as running many tasks over a list of items, with or without a concurrency limit. It was originally designed for Node.js, installed with npm, and can also be used directly in the browser. The README shows examples using Node-style callbacks and ES2017 async functions.
co
co is a JavaScript library that lets you write asynchronous code in a sequential style by running generator functions and resolving the values they yield, such as thunks and promises. The description says it supports thunks, promises, and similar patterns. It is aimed at Node.js developers who want to flatten callback-heavy or promise-chained code.
Best for
async
- JavaScript developers who need to run many asynchronous operations over a collection of items
- Node.js projects that use callback-style code and want helper functions for it
- Developers who want to limit how many asynchronous tasks run at the same time
- Projects bundling with Webpack or Rollup that can use the included ESM build
co
- Node.js developers maintaining code that already uses generator functions
- Developers reading or migrating older async code written around co-style generators
- Developers who want a minimal helper rather than a full framework
Strengths
async
- Provides ready-made helpers for common async patterns over collections, as shown in the README examples
- Works in Node.js and the browser, and includes an ESM/MJS build for modern bundlers
- Released under the MIT license with a large community (about 28,000 stars)
co
- Simple, focused purpose: one job of running generator-based async flow
- Widely starred on GitHub, indicating broad past adoption
- Released under the permissive MIT license
Not ideal for
async
- Developers working only with native Promises and async/await who may not need an extra library; the README does not compare these approaches
- Non-programmers, since the project is a code library with no end-user interface
- Projects that need documentation beyond the linked site, since the README itself points to external docs
co
- Projects using modern async/await, since native syntax covers the same need (general reasoning, not stated in the README)
- Anyone who needs documentation or a README that explains usage, since the provided README text is empty
Trade-offs
async
- The README is truncated and does not describe the full list of functions; the linked documentation holds the details
- The latest release listed is v2.3.0, while the docs link points to a v3 path, so version guidance is unclear from the README alone
- Examples mix callback and async-function styles, so beginners may need to learn both patterns
co
- The last push was in December 2020 and no releases are listed, so maintenance status is unclear
- The README provided here is empty, so usage details could not be verified
- Generator-based flow control has been largely superseded by native async/await in modern JavaScript
Getting started
Frequently asked questions
What is the difference between async and co?
async: A JavaScript utility library of helper functions for running asynchronous code in Node.js or the browser. co: A small JavaScript helper for running generator functions as async control flow. async suits javascript developers who need to run many asynchronous operations over a collection of items, while co suits node.js developers maintaining code that already uses generator functions.
Are async and co free?
async: Free and open source. co: Free and open source.
Is async or co more popular?
async has more GitHub stars (28.1k) than co (11.8k). Popularity isn't the same as fit — check which one matches your use case above.
Built from our catalogue entries for both products: open-source details come from each project's README (AI-assisted, refreshed weekly) and GitHub data; AI-tool details are editorial. Spotted something wrong? Let us know from either product's page.